Bandwidth limitations and signal presentation level variations affect the overall perception of speech quality. Presentation level is the active speech level at the listener side. How to measure active speech level is described in [1] ITU-T Rec. P. 56 (March 1993) Objective measurement of Active Speech Level.
If the bandwidth and the presentation level variations are the only source of degradation, they can be related in a simple way to speech quality; the signals with larger bandwidth and higher presentation level have higher quality and vice versa. However, in the case of typical coding artifacts, this relation becomes highly non-linear, and limiting the signal bandwidth and/or decreasing presentation level might lead to quality improvement. This effect is difficult to capture by the conventional quality assessment schemes, such as those disclosed in the following documents [2]-[6] below:
[2] ITU-T Rec. P.862 (February 2001), Perceptual evaluation of speech quality (PESQ), an objective method for end-to-end speech quality assessment in narrow-band telephone networks and speech codecs;
[3] ITU-T Rec. P.862.2 (November 2005), Wideband extension to Recommendation P.862 for the assessment of wideband telephone networks and speech codecs;
[4] ANSI T1.518-1998 (R2003), Objective Measurement of Telephone Band Speech Quality Using Measuring Normalizing Blocks;
[5] ITU-T P. 563 (May 2004), Single ended method for objective speech quality assessment in narrow-band telephony applications; and
[6] ITU-R Rec. BS.1387-1 (November 2001), Method for objective measurements of perceived audio quality.
Presentation level is related to the signal loudness, typically measured according to ITU-T Rec. P.56 speech level meter described in [1]. An example of a signal at different presentation levels is shown in FIG. 1 of this application.
Signal bandwidth is the range of frequencies beyond which the frequency function is close to zero (e.g. 10-20 dB below max frequency value). Example of a super-wideband signal (50-14000 Hz), processed with NB (narrowband) IRS (Intermediate Reference System) filter is given in FIG. 2. IRS defines sending/receiving characteristics of NB codecs and other NB systems. It defines a band-pass filter that attenuates below 300 Hz and above 3400 Hz and is described in [7] ITU-T Rec. P.48, Telephone Transmission Quality, Transmission Standards, Specification for an Intermediate Reference System.